1. Field of the Invention
The invention relates to welded tubing of a highly corrosion resistant ferritic stainless steel characterized by high resistance to hydrogen embrittlement. The tubing is adapted for use in heat exchangers handling process media containing hydrogen sulfide and other sources of nascent hydrogen, in cathodically protected heat exchangers, and in particular for cathodically protected heat exchangers operated at electrochemical potentials more negative than about -800 millivolts with respect to the saturated calomel reference electrode (SCE).
2. Description of the Prior Art
Heat exchangers and condensers are devices used to transfer heat from one medium to another. In the shell and tube type heat exchangers hot liquid or vapor is contained in the shell while the cool liquid passes through the tubes. For reasons of performance and economy, many electrical power plant condensers and chemical and petrochemical plant heat exchangers are now built with highly alloyed ferritic stainless steel tubing and with dissimilar metal tubesheets and water boxes. This practice is particularly common in rebuilt condensers or heat exchangers in which ferritic stainless steel tubing has been used to replace copper alloy tubing installed in tubesheets of matching or nearly matching compositions. Condensers and heat exchangers of this conventional construction are well known in the art as described in the following articles:
R. O. Miller and H. G. Seip, "Condensers and Heat Exchangers," Corrosion in Power Generating Equipment, Plenum Plublishing Corporation, 1984. PA1 G. V. Spires, et al, "Case History: Lining Condenser Components in a Nuclear Plant," Journal of Protective Coatings and Linings, 34-41, September 1984.
In an apparatus of this type where brackish or seawater cooling are used, the ferritic stainless steel tubing is required to be resistant on one surface to pitting and crevice corrosion as well as other forms of corrosive attack in these aggressive cooling waters and on the other surface to similar forms of corrosive attack from the process media. Since welding is used in either the construction or installation or both with respect to such tubing, the tubing must exhibit good weldability and be resistant to corrosion in the as-welded, and welded and annealed conditions. Also, the toughness and ductility of the tubing in these metallurgical conditions must be sufficient to avoid cracking during the expansion of the tubing into the tubesheets and in other forming operations.
For these applications, it is known to use ferritic stainless steels containing about 20 to 30% chromium and which are stabilized by the use of titanium, columbium, zirconium, and aluminum or combinations thereof. These stabilizing elements have generally been deemed to be equivalent for this purpose.
A significant disadvantage of welded tubing made from these conventional ferritic stainless steels is the susceptibility of the tubing to hydrogen embrittlement. This embrittlement, coupled with applied or residual stresses, can result in extensive cracking of the tubing. Hydrogen embrittlement has been found to be more pronounced in typical applications where the tubing is exposed to hydrogen sulfide or nascent hydrogen in the media being processed or where the heat exchangers are subjected to cathodic protection to minimize galvanic or crevice corrosion of the tubesheet or water box materials. As indicated in NACE Standard RP-01-69, it is conventional practice when cathodically protecting carbon steel or cast iron components in seawater cooled heat exchangers to utilize electrochemical potentials more electronegative than about -850 millivolts (SCE), whether the protection is afforded by use of sacrifical anodes or by automatic electrical systems. There is a need, therefore, for welded ferritic stainless steel tubing that exhibits high resistance to hydrogen embrittlement when exposed to process media containing hydrogen sulfide or other sources of nascent hydrogen or when used in a cathodically protected heat exchanger or condenser using brackish or seawater cooling.